Chlamydia traehomatis is a major cause of sexually transmitted disease and blindness. Development of an anti-chlamydial subunit vaccine appears feasible. Nearly all subunit vaccine studies have used the major outer membrane protein (MOMP) as the immunogen with only modest success. The identification of alternate surface proteins to test as vaccine candidates could significantly hasten the development of an effective vaccine. MicroCarb has developed a novel approach for identifying key surface adhesin proteins using various cell surface receptors. This approach was used with chlamydia to demonstrate specific binding of chlamydia to a distinctive cell membrane receptor component. Using this component as an affinity binding substrate, a novel membrane-associated protein, distinct from previously characterized chlamydial proteins, has been identified which binds this receptor. The proposed research will characterize this novel membrane protein with antisera generated against chlamydial membrane surface proteins of similar size. The antisera will be used to facilitate cloning of the gene encoding the receptor binding protein from a Chlamydia traehomatis gene bank generated in an E. coli expression system. Immunoreactive clones will be evaluated for expression of the novel receptor-binding protein and for their ability to absorb antibody that inhibits receptor binding and/or infectivity by C. traehomatis in vitro. Identification of a novel adhesin will permit evaluation of this protein as a subunit vaccine in a preclinical phase II application.